Download 9916575 1998-1999 Big Boss 6x6 Service Manual Chapter

CHAPTER 4
FUEL SYSTEM/CARBURETION
Jetting Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Main Jet Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fuel Tank Asm. Exploded View . . . . . . . . . . . . . . . . .
Fuel Flow Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Main Jet / Pilot Jet Part Numbers . . . . . . . . . . . . . . .
CV Carburetor System Function (4 Cycle) . . . . . . .
CV Carburetor Vent System (4 Cycle) . . . . . . . . . . .
CV Carburetor Operation . . . . . . . . . . . . . . . . . . . . . .
Exploded View, Mikuni BST 34 Carburetor . . . . . . .
Disassembly Notes, CV Carburetor . . . . . . . . . . . . .
Cleaning, CV Carburetor . . . . . . . . . . . . . . . . . . . . . .
Inspection, CV Carburetor . . . . . . . . . . . . . . . . . . . . .
Assembly, CV Carburetor . . . . . . . . . . . . . . . . . . . . . .
Adjustment, CV Carburetor . . . . . . . . . . . . . . . . . . . .
Fuel Pump Disassembly / Inspection / Assembly .
Fuel Pump Exploded View . . . . . . . . . . . . . . . . . . . . .
Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1
4.1
4.2
4.3
4.4
4.5
4.5
4.6-4.8
4.9
4.10-4.11
4.11
4.12
4.13
4.14-4.15
4.16
4.17
4.18-4.19
4
FUEL SYSTEM/CARBURETION
JETTING GUIDELINES
Changes in altitude and temperature affect air density, which is essentially the amount of oxygen available for
combustion. In low elevations and cold temperatures, the air has more oxygen. In higher elevations and higher
temperatures, the air is less dense.
Carburetors are calibrated for an altitude of 0-3000 ft. (0-900 meters) and ambient temperatures between +40
and +80 F (+5 to +26 C). Carburetors must be re-calibrated if operated outside the production temperature
and/or altitude range. The main jet installed in production is not correct for all altitudes and/or temperatures. In
addition, air screw / pilot screw adjustments may be required to suit operating conditions.
CAUTION:
A main jet that is too small will cause a lean operating condition and may cause serious engine damage.
Select the correct main jet carefully for elevation and temperature according to the charts in the General/
Specifications Chapter, or in the Owner’s Safety and Maintenance Manual for each particular model.
MAIN JET SELECTION
IMPORTANT: The following guidelines must be followed when establishing a main jet setting:
1.
2.
3.
4.
Select the lowest anticipated temperature at which the machine will be operated.
Determine the lowest approximate altitude at which the machine will be operated.
Select the correct main jet from the chart.
Clutching changes may also be required for changes in elevation. Refer to clutching charts in General /
Specifications Chapter 1 for recommendations.
WARNING
Gasoline is extremely flammable and explosive under certain conditions.
Always stop the engine and refuel outdoors or in a well ventilated area.
Do not overfill the tank. Do not fill the tank neck.
Never start the engine or let it run in an enclosed area. Gasoline powered engine exhaust fumes are poisonous and can cause loss of consciousness and death in a short time.
Never drain the float bowl when the engine is hot. Severe burns may result.
Do not smoke or allow open flames or sparks in or near the area where refueling is performed or where
gasoline is stored.
If you get gasoline in your eyes or if you should swallow gasoline, see your doctor immediately.
If you spill gasoline on your skin or clothing, immediately wash it off with soap and water and change clothing.
4.1
FUEL SYSTEM/CARBURETION
FUEL TANK ASSEMBLY
Forward
Fuel tank mounted valve
4.2
FUEL SYSTEM/CARBURETION
FUEL FLOW DIAGRAM
Fuel Tank Vent
Fuel Tank
Fitting Screens
L Fuel Pump
Filter
Fuel Valve
Carburetor Vent
Carburetor
Engine
L Located Above Oil Tank
4.3
FUEL SYSTEM/CARBURETION
MIKUNI JET PART NUMBERS
Main Jet Part Numbers
Mikuni BST Carburetor
Jet Number
112.5
115
117.5
120
122.5
125
127.5
130
132.5
135
137.5
140
142.5
145
147.5
150
152.5
155
157.5
160
162.5
165
167.5
170
Part Number
3130554
3130555
3130556
3130557
3130558
3130559
3130560
3130561
3130562
3130563
3130564
3130527
3130566
3130567
3130568
3130569
3130570
3130571
3130572
3131141
3131142
3131143
3131144
3131145
Pilot Jet Part Numbers
Mikuni BST Carburetor
Jet Number
40.0
42.5
4.4
Part Number
3130624
3130526
FUEL SYSTEM/CARBURETION
CV CARBURETOR SYSTEM FUNCTION
Carburetor Component Function
System
Main Components
Main Function
Main Affect
Float System
(Fuel Level Control)
Inlet Pipe, Needle and Seat,
Float, Float Pin
Maintains specified fuel level
in float chamber (carburetor
float bowl)
All systems
All throttle ranges
Venting
Vent Passages in Carburetor,
Vent lines (2) into (1) to frame
Supplies atmospheric pressure to fuel in float chamber
All systems
All throttle ranges
Starter
(Choke/Enrichment)
Choke Lever, Cable, Choke
Supplies additional fuel air
Plunger, Return Spring, Carb mixture necessary for cold
Passages (Starter Jet, Starter starting
Bleed Pipe)
All throttle ranges
Greatest effect at low throttle
settings and idle
Pilot (Idle System)
Pilot Jet/Passageways, PilotMixture Screw with Spring
Washer and Sealing O-Ring,
Bypass Ports (Behind
Throttle Plate), Pilot Air Jet,
Pilot Outlet, Throttle Plate
Primarily supplies fuel at idle
and low throttle settings
Mainly idle to 1/4 throttle
Minimal effect after 1/2
throttle
Main System
Main Jet, Main Air Jet, Main
Air Passage, Needle Jet, Jet
Needle, Vacuum Slide,
Throttle Plate
Supplies fuel at mid-range
and high throttle settings.
1/4 to full throttle
VENT SYSTEMS - CV CARBURETOR
The carburetor float bowl vent lines supply atmospheric
pressure to the fuel in the float bowl. The lines must be
free of kinks and restrictions and be properly routed to allow fuel to flow in the proper amount and to prevent contaminants from entering the carburetor.
To frame
Vent lines
4.5
FUEL SYSTEM/CARBURETION
MIKUNI CV CARB OPERATION
The constant velocity carburetor incorporates a mechanically operated throttle plate and a vacuum controlled
slide valve (vacuum slide). The venturi cross-sectional area in the carburetor bore is increased or decreased
automatically by the vacuum slide, which moves according to the amount of negative pressure (less than atmospheric) present in the venturi.
A diaphragm attached to the top of the vacuum slide is sealed to the slide and to the carburetor body forming two
chambers. The chamber above the diaphragm is connected to the venturi area by a drilled orifice in the center
of the vacuum slide. The chamber below the diaphragm is vented to atmospheric pressure by a passage on the
air box side of the carburetor. A spring, installed in the center of the vacuum slide, dampens the slide movement
and assists the return of the slide.
When the throttle plate is opened and engine speed begins to increase, the pressure in the venturi (and therefore
in the chamber above the diaphragm) becomes significantly lower than atmospheric. Atmospheric pressure in
the chamber below the diaphragm forces the diaphragm upward, raising the slide against spring pressure. When
the pressure above and below the diaphragm are nearly equal, the slide moves downward under spring pressure.
Raising or lowering the slide increases or decreases the cross sectional area in the venturi, and therefore the air
velocity in the venturi is kept relatively constant. This provides improved fuel atomization and optimum fuel/air
ratio.
Diaphragm
Diaphragm
Low Pressure
From Venturi
Low Pressure
From Venturi
Vacuum
Slide
Air Box
Pressure
From
Air Box
From
Air Box
Vacuum
Slide
Throttle
Plate
Throttle
Plate
Venturi
Air Flow
Air Flow
Low Pressure
Low Pressure
Note: Diagrams are for explanation of theory only, and are not true representations of Mikuni BST 34 carburetor.
4.6
FUEL SYSTEM/CARBURETION
PILOT (IDLE AND SLOW) SYSTEM
This system supplies fuel during engine
operation with throttle valve closed (1) or
slightly opened. The fuel from float chamber (2) is metered by pilot jet (3) where it
mixes with air coming in through pilot air
jet (4). The mixture then goes up through
pilot passage to pilot screw (5). A part of
the mixture is discharged into the main
bore out of bypass ports (6). The remainder is then metered by pilot screw and discharged into the main bore through pilot
outlet (7).
4
1
6
7
5
2
3
STARTER SYSTEM (CHOKE OR ENRICHMENT)
When the choke cable (1) is activated, the
starter plunger (5) is lifted off the seat.
Fuel is drawn into the starter circuit from
the float chamber (2) through the starter
jet (3). Starter jet meters this fuel, which
then flows into starter pipe (4) and mixes
with the air (7) coming from the float
chamber. The mixture, rich in fuel content, reaches starter plunger and mixes
again with the air coming through a passage (8) extending from underneath the
diaphragm. The rich fuel/air mixture for
starting is discharged through starter outlet (6) in the the main bore.
8
5
1
6
7
4
2
3
4.7
FUEL SYSTEM/CARBURETION
FLOAT SYSTEM
Fuel enters the float chamber (3) by
means of the inlet pipe and passage,
through a screen on the back of the inlet
needle seat (4), and around the inlet
needle (2). As the fuel fills the float chamber, the float (1) rises and forces the inlet
needle against the seat, shutting off the
orifice in the seat. When fuel level is up in
float chamber, floats are up and needle
valve remains pushed up against valve
seat. Under this condition, no fuel enters
the float chamber. As the fuel level falls,
floats go down and needle valve unseats
itself to admit fuel into the chamber. In this
manner, the needle valve admits and
shuts off fuel alternately to maintain a
practically constant fuel level inside the
float chamber.
Inlet
Pipe
1
3
4
2
MAIN SYSTEM
As throttle valve (1) is opened, engine
speed rises, and this increases negative
pressure in the venturi. Consequently the
vacuum slide (2) moves upward. The fuel
in float chamber (3) is metered by main jet
(4), and the metered fuel enters needle jet
(5), in which it mixes with the air admitted
through main air jet (6) to form an emulsion. The emulsified fuel then passes
through the clearance between needle jet
(5) and jet needle (7), and is discharged
into the venturi (A). Mixture proportioning
is accomplished in needle jet (5); the clearance through which the emulsified fuel
must flow is determined ultimately by
throttle position and vacuum slide height.
2
1
7
6
A
3
5
4
4.8
FUEL SYSTEM/CARBURETION
BST 34 CARBURETOR EXPLODED VIEW
1. Carburetor Assembly
2. Screw
3. Throttle Valve
4. Cover, Diaphragm
5. Jet Block Assembly
6. Spring
7. Diaphragm Assembly
8. Ring
9. “E” Ring
10. Ring
11. Needle Jet
12. Cover
13. O-Ring
14. Throttle Shaft Assembly
15. Ring
16. Seal
17. Spring
18. Packing
19. “E” Ring
20. Cap
21. Screw
22. Drain Screw
23. O-Ring
24. Washer
25. Adjuster
26. Spring
27. Pilot Jet
28. Main Jet
29. Washer
30. Jet Needle
31. Float Assembly
32. Float Body Assembly
33. Float Pin
34. Needle Valve
35. O-Ring
36. O-Ring
37. Filter
38. Screw
39. Screw
40. Screw
22
1
3
5
Refer to Page 4.4 for Jet Part Numbers
Jet Needle
“E” Clip Position
2
41. Guide Holder
42. Spring
43. Plunger Assembly
4 44. Spring Washer
45. Screw
46. Air Jet
47. Cable Guide
48. Spring
49. Ring
50. Adjust Screw
51. Screw and Washer Assy.
4.9
FUEL SYSTEM/CARBURETION
CARBURETOR DISASSEMBLY - MIKUNI CV
Use the following disassembly, assembly, and inspection techniques to service a CV carburetor.
1. Remove carburetor diaphragm chamber cover with
a ratchet style screwdriver. DO NOT use an impact
driver to remove the screws or carburetor may be
permanently damaged.
2. Use a small spring loaded center punch to remove
pressed float pin.
NOTE:Set the center punch to the softest setting (if adjustable) to avoid damage to float pin tower.
3. Remove inlet needle seat retaining screw along with
plate, and carefully remove needle seat. NOTE: Do
not use a pliers to remove the seat or permanent
damage may occur.
4.10
FUEL SYSTEM/CARBURETION
CARBURETOR DISASSEMBLY - MIKUNI CV, CONT.
4. Do not misplace the pilot mixture screw, spring, flat
washer, or O-Ring. If anti-tamper plug is installed in
pilot screw cavity, refer to Maintenance chapter 2 for
removal procedure.
O-Ring
Washer
Spring
Pilot Screw
5. NOTE: The starter jet is not removeable.
Starter Jet
CARBURETOR CLEANING
1. Thoroughly clean the carburetor body, jets, and all passages with carburetor cleaner or electrical contact
cleaner.
WARNING
Protect eyes from contact with cleaner. If you get cleaner in your eyes or if you swallow cleaner, see your doctor
immediately. Some carburetor cleaners are extremely caustic and extended periods of soaking can loosen the
adhesive sealer on the passage drill-way plugs. Do not soak rubber or plastic components (such as the vacuum
slide diaphragm, needle seat screen, or O-Rings in caustic cleaning solutions. Irreparable damage may occur.
Do not use agitator type carburetor cleaning equipment. Rubber parts must be cleaned with mild detergent and
hot water only.
2. If the carburetor is extremely dirty or contaminated with fuel residue and varnish, soak for short periods only in
carburetor cleaner, and rinse in hot water.
3. Replace the jets if they are extremely dirty or have a buildup of fuel residue or bacterial growth. Even a small
amount of residue will reduce the flow characteristics of the jet.
4. Verify all passages and jets are unobstructed by spraying electrical contact cleaner through the passages.
CAUTION: Do not use wire or welding tip cleaners on the jets as the orifice size may be altered.
5. Use low pressure air to dry carburetor body and all components.
4.11
FUEL SYSTEM/CARBURETION
CARBURETOR INSPECTION
1. Inspect jet needle and needle jet for wear. Look for
discoloration, shiny spots, or and area that looks
different than the rest of the needle. The middle to
upper portion of the needle where it contacts the
needle jet is the most likely wear point. If jet needle
shows signs of wear replace both needle and needle
jet to prevent a rich condition.
Inspect this
area
2. Inspect the inlet needle tapered surface for any sign of
wear or damage. Be sure the spring loaded pin is free
moving and returns freely when pushed. The inlet
needle and seat should be pressure tested after
assembly.
Good Condition
Worn, Deposits
Seat
Wear areas
Needle
4.12
FUEL SYSTEM/CARBURETION
CARBURETOR ASSEMBLY
1. Inspect the diaphragm (A) for holes,
deterioration, or damage. Make sure the
diaphragm is pliable but not swollen. The
diaphragm should fit properly in the carburetor
body.
Replace diaphragm assembly if
diaphragm is damaged.
2. Replace parts in proper order. The spring seat
washer (B) is stepped and must be placed on
TOP of “E” Clip (C). Spacer washer (D) must
be installed below the E-Clip. Refer to parts
manual for more information.
3. Be sure the tab (E) on outer edge of
diaphragm is positioned properly in the
carburetor body.
B
D
E
C
A
4. Install the pilot mixture screw, spring, washer, and
O-ring as an assembly. Lubricate the O-Ring with oil or
light grease before installation. CAUTION: Do not
damage the O-ring during installation. Turn the screw
in until it lightly contacts the seat. Back out the
specified number of turns. NOTE: The final pilot (idle)
mixture must be adjusted with the engine running.
Refer to Page 2.13.
Pilot Mixture Screw Base Setting
(Turns Out)
Refer to General / Specifications
Chapter 1
Pilot Screw
4.13
FUEL SYSTEM/CARBURETION
FLOAT HEIGHT ADJUSTMENT
1. Place the carburetor on a level surface as shown at
right to remove weight from float arm. In this position,
the float tongue will rest lightly on the inlet needle valve
pin without compressing the spring.
2. Measure the height from the float bowl mating surface
to the top of step in float as shown. Both sides of float
should be parallel to each other. The measurement
should be made at the mid-point on the top of the float
using float adjustment tool (PN 2872314) or a vernier
caliper. When measuring the height be sure the inlet
needle valve spring is not compressed.
Float Height:
Std: BST 34
14.7mm (.58)  1 mm
3. If adjustment is necessary, bend the tongue slightly.
Be sure float arms are even on left and right side.
Float arms even
Bend to adjust
float
4.14
FUEL SYSTEM/CARBURETION
NEEDLE AND SEAT LEAKAGE TEST
1. Install the float bowl. Invert the carburetor and install a
Mity-Vac (PN 2870975) to the fuel inlet fitting. Apply
5 PSI pressure to inlet fitting. The needle and seat
should hold pressure indefinitely. If not, inspect needle
and seat and seat O-ring.
Mity Vac PN 2870975
FUEL LEVEL
A fuel level test can be performed on some models if the
drain hose fitting is accessible. Be sure to re-attach the
bowl drain hose after performing the test. A fuel level test
allows you to observe the height of the fuel in the float bowl
without removing the carburetor. The fuel level can be observed with the engine either running or shut off, however,
engine must run briefly to allow fuel level to stabilize..
1. Attach a clear line to drain fitting. Be sure line fits tightly
on fitting. Position hose along side of carburetor as
shown.
2. Open bowl drain screw by turning counterclockwise
approximately two turns. Start and run engine for 3 to 5
seconds to allow fuel level to stabilize in the line. If level
is out of specification, remove carburetor and inspect
inlet needle and seat, float height, passages, etc.
1.5 mm
.060
Bowl Mating
Surface
3. If a line was removed to perform this procedure, it must
be replaced.
4.15
FUEL SYSTEM/CARBURETION
FUEL PUMP
The Polaris 6x6 is equipped with a pressure regulated fuel pump (about 1-3 PSI). The pump is located under
the headlight cover at the front of the machine. Refer to illustration on following page for fuel pump component
identification.
To test the fuel pump:
1. Turn fuel off.
2. Disconnect impulse line from pump.
3. Connect Mity-Vac (PN 2870975) to the impulse line fitting on the pump.
4. Apply 5 inches (Hg) vacuum to the pump fitting. The diaphragm should hold vacuum indefinitely.
If fuel is present in the impulse line or vacuum chamber of the pump, the diaphragm is ruptured and the pump
diaphragms must be replaced.
FUEL PUMP DISASSEMBLY
1.
2.
3.
4.
Refer to illustration on following.
Remove the screws from the pump diaphragm cover. Note the location of the two longer screws.
Remove the diaphragm cover gasket, diaphragm, and valve body gasket.
Remove the outlet check valve cover, diaphragm, and gasket.
FUEL PUMP INSPECTION/ASSEMBLY
1. Inspect inlet and outlet check valves for cracks,
warpage or damage. Inspect the diaphragms for
cracks, holes or swelling.
2. To clean the valves or pump body, remove the set
screw and washer. Remove the valve and wash
with soap and water. Carburetor cleaner may be
used to clean the pump body when the check valves
are removed.
CAUTION:
Some carburetor
cleaners are very caustic and should not be used to
clean the non-metal parts of the fuel pump.
3. Check the sealing surfaces of the pump body and
covers. Carefully remove all traces of old gasket
and check the surfaces for damage. Replace
diaphragms and gaskets as a set.
4. Reassemble the pump in the reverse order of
disassembly. Tighten all screws evenly.
4.16
FUEL SYSTEM/CARBURETION
FUEL PUMP EXPLODED VIEW
4
5
1. Fuel Pump Assembly
2. Diaphragm, Gasket Set
3. Screw and Washer Assembly
4. Screw and Washer Assembly
5. Screw and Washer Assembly
6. Pressure Regulator
7. Fuel Inlet
8. Fuel Outlet
5
3
2
1
8
7
6
4.17
FUEL SYSTEM/CARBURETION
TROUBLESHOOTING
Fuel Starvation/Lean Mixture
Symptoms: Hard start or no start, bog, backfire, popping through intake / exhaust, hesitation, detonation, low
power, spark plug erosion, engine runs hot, surging, high idle, idle speed erratic.
S No fuel in tank
S Restricted tank vent, or routed improperly
S Fuel lines or fuel valve restricted
S Fuel filter plugged
S Carburetor vent line(s) restricted
S Plugged or restricted inlet needle and seat screen or inlet passage
S Clogged jets or passages
S Float stuck, holding inlet needle closed or inlet needle stuck
S Float level too low
S Fuel pump inoperative
S Air leak at impulse line
S Restricted impulse line (kinked, pinched)
S Intake air leak (throttle shaft, intake ducts, airbox or air cleaner cover)
S Ruptured vacuum slide diaphragm, Vacuum slide stuck closed or sticky
S Improper spring
S Jet needle position incorrect
S Incorrect pilot screw adjustment
Rich Mixture
Symptoms: Fouls spark plugs, black, sooty exhaust smoke, rough idle, poor fuel economy, engine runs rough/
misses, poor performance, bog, engine loads up, backfire.
S Air intake restricted (inspect intake duct)
S Air filter dirty/plugged
S Choke plunger sticking, incorrectly adjusted choke
S Choke cable binding or improperly routed
S Incorrect pilot air/fuel screw adjustment
S Faulty inlet needle and seat
S Faulty inlet needle seat O-Ring
S Float level too high
S Poor fuel quality (old fuel)
S Loose jets
S Worn jet needle/needle jet or other carburetor parts
S Dirty carburetor (air bleed passages or jets)
S Weak or damaged vacuum piston return spring
S Fouled spark plug
Poor Idle
Symptoms: Idle too high.
S Idle adjusted improperly/idle mixture screw damaged
S Sticky vacuum slide
S Throttle cable sticking, improperly adjusted, routed incorrectly
S Choke cable sticking, improperly adjusted, routed incorrectly
4.18
FUEL SYSTEM/CARBURETION
TROUBLESHOOTING, CONT.
Idle Too Low
S Choke cable bending or incorrectly adjusted
S Idle speed set incorrectly
S Idle mixture screw misadjusted or damaged
S Belt dragging
S Ignition timing incorrect
S Worn jet needle/needle jet
Erratic Idle
S
S
S
S
S
S
S
S
S
S
S
S
Choke cable bending or incorrectly adjusted
Throttle cable incorrectly adjusted
Air leaks, dirty carburetor passages (pilot circuit)
Pilot mixture screw damaged or adjusted incorrectly
Tight valves
Ignition timing incorrect
Belt dragging
Dirty air cleaner
Engine worn
Spark plug fouled
Idle speed set incorrectly (speed limiter)
Worn jet needle/needle jet
4.19
NOTES